fix: re-validate outbound connections at fetch time against internal hosts

validate_url only inspects the submitted URL string. yt-dlp then follows HTTP
redirects and resolves media URLs from remote metadata without re-checking, so
an allowed URL that 302s to http://169.254.169.254/ (cloud metadata) or an
RFC1918 host is still fetched — the guard's own docstring scoped this out.

Install a getaddrinfo guard in the download subprocess that re-validates every
resolved address at actual connect time, covering redirects and DNS rebinding
for any backend resolving through Python's socket module (urllib, requests).
Loopback is permitted so locally-configured proxies keep working; link-local,
RFC1918 and unique-local are blocked. Native resolvers (curl_cffi/libcurl via
--impersonate) bypass this and rely on network isolation as the backstop.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/ytdl.py

@@ -26,7 +26,7 @@ from dl_formats import get_format, get_opts, AUDIO_FORMATS, merge_ytdl_option_la
 from datetime import datetime
 from state_store import AtomicJsonStore, from_json_compatible, read_legacy_shelf, to_json_compatible
 from subscriptions import _entry_id
-from url_guard import validate_url
+from url_guard import validate_url, install_socket_guard
 
 log = logging.getLogger('ytdl')
 
@@ -635,6 +635,10 @@ class Download:
                 os.setpgrp()
             except OSError:
                 pass
+        # Re-validate every outbound connection at fetch time. validate_url only
+        # saw the submitted URL string; this catches redirects and DNS rebinding
+        # to internal hosts (cloud metadata, RFC1918) that it cannot.
+        install_socket_guard()
         log.info(f"Starting download for: {self.info.title} ({self.info.url})")
         try:
             debug_logging = logging.getLogger().isEnabledFor(logging.DEBUG)